1. Technical Field
The disclosure is related to a pixel driving circuit of an organic light emitting diode, and more particularly, to a pixel driving circuit of an organic light emitting diode for compensating a threshold voltage of a transistor.
2. Related Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional display panel utilizing organic light emitting diodes (OLED). The display panel 10 comprises a data driver 11, a scan driver 12 and a display matrix 13. The data driver 11 controls data lines DL1 to DLn, and the scan driver 12 controls scan lines SL1 to SLm. The data lines DL1 to DLn and the scan lines SL1 to SLm are interlaced to form the display matrix 13. Each interlaced data line and scan line forms one display unit. For instance, the data line DL1 and the scan line SL1 form the display unit 14. As shown in FIG. 1, an equivalent circuit of the display unit 14 (which is similar to other display units) comprises a switching transistor T11, a storing capacitor C11, a driving transistor T12 and an organic light emitting diode D11, where the switching transistor T11 and the driving transistor T12 are N-type transistors.
The scan driver 12 outputs scan signals to the scan lines SL1 to SLm sequentially, so only the switching transistors corresponding to display units of a certain row of the scan driver 12 are turned on at one time, while switching transistors corresponding to display units of other rows are turned off. The data driver 11 outputs video signals (e.g. grey level values) to one row of display units via data lines DL1 to DLn, according to an image data to be displayed. For instance, when the scan driver 12 outputs the scan signal to the scan line SL1, the switching transistor T11 of the display unit 14 is turned on, the data driver 11 transmits a corresponding pixel data to the display unit 14 via the data line DL1, and a voltage of the pixel data is stored in the storing capacitor C11. The driving transistor T12 provides a driving current Ids to drive the organic light emitting diode D11 according to the voltage stored in the storing capacitor C11.
Since the organic light emitting diode D11 is a current driven component, the value of the driving current Ids determines a brightness of the light emitted by the organic light emitting diode D11. The driving current Ids, equivalent to a current flowing through the driving transistor T12, can be represented by formula (1):
                    Ids        =                              1            2                    ⁢                                    k              ⁡                              (                                  Vgs                  -                  Vth                                )                                      2                                              (        1        )            where k represents a conducting parameter of the driving transistor T12, Vgs represents a voltage difference between a gate end and a source end of the driving transistor T12, and Vth represents a threshold voltage value of the driving transistor T12.
However, due to process variables of thin film transistors, electrical characteristics are varied for driving transistors in different regions of the display matrix 13, meaning threshold voltage values for the driving transistors are different. Therefore, when display units in different regions receive pixel data of the same voltage, values of the driving current provided to the organic light emitting diodes of the display units may be inconsistent, due to the threshold voltage difference between corresponding driving transistors. Consequently, varying brightness is generated by the organic light emitting diodes, causing the display panel 10 to display non-uniform images.